Toyota ZZT230 series Repair Manual page 289

Dl-78
DTC P0420
CIRCUIT DESCRIPTION
The compar e s the wavef o rm of t h e oxygen sensor l o cat e d bef o re t h e cat a l y st wi t h t h e wavef o r m of
the
ECM
oxygen sensor l oc at e d behi n d t h e cat a l y st t o det e rmi n e whet h er or not cat a l y st perf o rmance has det e ri o �
r a t ed . Air -f uel r a ti o f e edback compensat i o n keeps the wavef o r m of the oxygen sensor bef o r e t h e cat a l y st r e peat ·
edl y changi n g back and f o r t h f r om r i c h t o l o an.
tf
cat a l y st i s f u nctioni n g normal l y ,
rich
the
and
f o rth bet w een and l e an much more sl o wl y t h an t h e wavef o rm of t h e oxygen sensor bef o r e t h e cat a -
lyst.
But
wavef o rms change at a si m i l a r r a t e , i t i n di c at e s t h at cat a l y st per f or m ance has det e ri o rat e d.
when
both
of
Heated Oxygen
Waveform before C&t a l y st
Sensor
DTC No.
driven
engine
After the
sensor
vehicle is
.
P0420 the waveforms of
... ....
2)
bank 1
2
Catalyst System Efficiency Below Threshold
wavef o rm of t h e oxygen sensor behi n d t h e cat a l y st swi t c hes back
the
OTC Detecting Condition
and the catalyst are warmed up. and
the
within the set vehiCle and engine speed range,
oxygen sensors (bank 1 sensor 1 and
amplitude
have the same
-
ENGINE
Normal Catal y st
..
the
while
• Gas
leakage on exhaust system
•Open or short in heated oxygen sensor circuit
Heated oxygen sensor
•
Three-way catalytic converter
•
Waveform of Heated Oxygen
behind Catal y st
Sensor
Trouble Area
-
F17081